1. Field of the Invention
The present invention generally relates to storage devices, and more particularly to a storage device which performs a data recording/reproducing operation on a disk-shaped medium while a head seek operation is performed on the basis of control information recorded on the disk-shaped medium.
Recently, the recording density of storage devices as described above has been increased and various improvements have been proposed and employed. Examples of such improvements are a data-recorded-surface servo for compensating for off-track of a head with respect to the disk medium, a magnetoresistive effect (MR) head, and ID-less (ID is information for writing and reading) format. When the MR head is used as a data head, the characteristic of the MR head is varied each time the write operation is performed. If a read error occurs, a dummy write is executed as a retry process, so that the characteristic of the MR head is recovered to the normal state. A process is also employed in which servo information is written into a guard band of the disk medium in order to prevent an erroneous detection of an index due to write noise.
2. Description of the Related Art
A magnetic disk drive is one of the storage devices, and a predetermined number of magnetic heads provided to a predetermined number of magnetic disks. In a seek operation, the magnetic heads are moved in the radial direction of the magnetic disks, and information recording/reproducing operations are carried out on the target cylinders. A seek control is performed in order to correctly position the magnetic heads on the target cylinders. In such a seek control, a closed-loop servo control is performed while servo information recorded on the magnetic disks beforehand is read. Such a servo control is classified into a servo-surface servo or a data-recorded-surface servo. In the servo-surface servo, servo information is recorded on a specific servo surface. In the data-recorded-surface servo, servo information is recorded on a data surface.
FIGS. 1A and 1B show a conventional magnetic disk related to the servo-surface servo. More particularly, FIG. 1 shows a servo disk 11, and FIG. 1B shows a data disk 21. In the servo-surface servo, a single servo disk and a predetermined number of data disks are used.
The servo disk 11 shown in FIG. 1A records servo information for the seek operation. A servo area on which servo information is recorded is divided, according to combinations shown in FIGS. 3A and 3B, into an outer guard band OGB, a data area and an inner guard band IGB. A first OGB (Outer Guard Band) area 13 and a second OGB area 14 are arranged further out than a data area 12. First outer guard band information is recorded on the first OGB area, and second outer guard band information is recorded on the second OGB. An IGB (Inner Guard Band) area 15 on which inner guard band information is recorded is located further in than the servo area 12.
The data disk 21 shown in FIG. 1B has a data area 22 on which data is recorded. A first OGB area 23 and a second OGB area 24 are located further out in a radial direction from a center of the data disk 21 than the data area 22. Servo information for correcting an off-track due to errors of the positions of the magnetic heads is recorded on the first OGB area 23. No information is recorded on the second OGB area 24. An IGB area 25 on which no information is recorded is located further in than the data area 22 in a radial direction from a center of the data disk 21.
The data-recorded-surface servo does not use the servo disk 11 shown in FIG. 1A, but uses a predetermined number of data disks 21 only. On the data disk, provided are servo information areas 26 which extend radially from the disk center. On the servo information areas 26, recorded are information concerning sectors, cylinders, head addresses, indexes and guard band information.
A single index is recorded on each of all the cylinders (including the OGBs) of the servo disk 11 in the servo-surface servo. A single index is recorded on each of the cylinders (including the OGBs) of each data disk in the data-recorded-surface servo. Surface analysis information, which may include information of a defective sector and various types of information, is recorded on a predetermined number of cylinders located further in than the data area of each data disk irrespective of whether the servo is the servo-surface servo or the data-recorded-surface servo.
The above-mentioned structure in which the servo information is recorded on the first OGB areas 13 and 23 of the magnetic disks 11 and 21 in the servo-surface servo and the data-recorded-surface servo is disclosed in Japanese Laid-Open Patent Application Nos. 52-46805, 4-40678, 4-205977 and 5-225725. Further, Japanese Laid-Open Patent Application No. 2-10601 discloses that the first OGB area 23 of the data disk is used as a data area, and can also be used as an effective data area.
The index is detected in the magnetic disk drives using the servo disk 11 and the data disks 21 related to the servo-surface servo or the data disks related to the data-recorded-surface servo. The servo control directed to positioning the magnetic heads on the target cylinders is carried out with respect to the position of the detected index. In this way, the recording/reproducing operation can be performed.
When an MR head is used as a data head as has been described previously, a read error may take place because the head characteristic is varied each time the write operation is performed. In this case, a dummy write operation on a specific dummy write cylinder is performed as a retry process after the occurrence of the read error. Hence, the head characteristic can be recovered to the normal one. In this case, the dummy write operation is carried out with respect to the data cylinders and the first OGB area 23 on each of the data disks 21 irrespective of whether the servo is the data-recorded-surface servo or the servo-surface servo.
FIG. 2 is a timing chart of the conventional dummy write operation. More particularly, FIG. 2 shows a dummy write operation on the first OGB area 23 in the servo-surface servo. Index information (index pattern "01011") of a cylinder on the servo disk 11 on which the dummy write operation is carried out is detected. Then, a write gate is maintained in the open state until the next GB information over the first GB information (GB pattern "01010") after the detection of the above index information. In this state, the dummy write operation is performed. After the dummy write operation, the servo information concerning the data surface for the off-track correction is read.
However, in the dummy write operation in the data-recorded-surface servo, the ID information cannot be correctly read in such a state where a read error occurs. Hence, it is not possible to determine whether the data head is correctly positioned on the dummy cylinder. Hence, there is a possibility that the dummy write operation may be carried out on a data cylinder and data may be deleted.
In the following case, servo information concerning the data surface for the off-track correction and/or the surface analysis information may be deleted if the cylinder address has an error. That is, an area subjected to the dummy write operation is provided in the first OGB area 23 or the area on which the surface analysis information is recorded in order to perform the reading of the dummy write operation and reading of the servo information and the surface analysis information with respect to the same cylinder. In this case, the dummy write can be unconditionally performed.
When the servo information is written or the dummy write operation is performed in the servo-surface servo, the index pattern, e.g. "01011" and the GB information, e.g. "01010" are set, as shown in FIG. 2.
FIGS. 3A and 3B show bit patterns. More particularly, FIG. 3A shows bit "0", and FIG. 3B shows bit "1". When a missing pulse as indicated by the broken line in FIG. 3B is detected with respect to bit "1", the bit pattern becomes bit "0" shown in FIG. 3A. The difference between the index pattern and the GB pattern is one bit (normally, the difference is represented by one bit or two bits). Hence, if the missing occurs due to write noise, the GB pattern is mistakenly detected as the index pattern. This problem is serious in not only the dummy write operation but also the recording of the data-recorded-surface servo information.
If the index pattern is mistakenly detected when the dummy write operation is carried out on the first OGB area 23 of the servo disk 11 on which the servo information is recorded, the servo information is deleted.